There is a need for a technically safe and continuous monitoring of the oil flow by detecting a hose failure or a limited leak during buoy loading.
On Wednesday the 12th of December 2007, large quantities of crude oil were spilt during buoy loading of the tanker vessel Navion Britannica at Statfjord A in the North Sea. The spill was a consequence of a failure in the upper part of the loading buoy system (OLS) in the hose running from the buoy to the vessel. The loading system can transfer up to 7500 m3 of oil per hour. Statoil has stated that pumping of the oil to sea went on for 45 minutes, and that the spill amounted to about 4000 m3. The system was shut down when one caught sight of the oil flowing up at the vessel.
An OLS system consists of a base frame connected to the platform via a pipeline. A vertical riser runs from the base frame and up to a swivel being held up by a floating element. A flexible pipe runs from the swivel to another swivel having another flexible riser, which is pulled into the bow of the tanker. Coflexip supplies the flexible risers. A valve, which may be released hydroaccoustically, is installed at the bottom of the vertical riser to shut down oil loading if a leak should arise.
A filling operation may take up to twenty hours for the largest shuttle tankers, and extra vigilance is needed to ensure that there are no leaks to sea. Visual inspection is part of the routines, but this is problematic when it is dark. The loading operations are manually monitored by comparison between measurements on the platform and on the ship. The OLS system is also monitored by means of pressure sensors to detect leaks.
The loading operations offshore are manual. The measurements on the ship and on the platform are separate systems, and comparison does not take place continuously.
The comparison is thereby not reliable since the measurements may vary, even if they should be equal over time.
The OLS system is monitored by means of pressure sensors to detect leaks, but the oil has a very low pressure as it enters the tanker, and a pressure drop in the oil flow due to leaks is therefore difficult to detect.
The volumes being filled and emptied on the platform and the ship respectively is monitored. There is a telemetric communication between tanker and platform to transmit these data so that differences may be discovered. This did however not function satisfactorily in the incident at the Statfjord field.
There exist leakage detection systems for subsea installations. The system consists of passive and active sonar. Leaks give off sound that may be detected by passive sonar. The active sonar detects bubbles as a consequence of gas leaks. The system functions at distances of 500 m or more. No such monitoring system has been supplied for subsea loading buoys, but such systems are used on pipelines, and they may also be used in loading operations. StatoilHydro has installed such leakage detection systems at Troll Pilot, Tordis and Ormen Lange. The complexity and cost are higher than for the invention applied for.
From the patent literature as background technology is put forward:                US 2007131297 A1 describes a method for monitoring a hydro carbon liquid stream flowing through a pipe combined with a two-layer hose for liquid transport offshore, where an optical sensor detects any leak collecting in an empty space at the hose end terminal if leaks from the inner layer flow into the interstice between the inner and outer hose layer. Reports may be given automatically via telemetry to a monitoring station. The invention works on a different principle than that described in this application, as this also functions through comparison of two measurements to indicate leaks between the measuring points.        NO 180132 B describes an external combined protection and leak indication device for a circular flange connection, and differs considerably from the invention applied for both regarding object and embodiment. The device measures leakage directly at a leakage point, while the invention applied for compares two measurements to detect leakage between the measuring points.        NO 317072 B1 describes monitoring of a pipeline system and is intended for another use without a description of the physical design of the measuring device.        